1. Field of the Invention
The present invention relates to a method for making an inflation film.
2. Description of the Related Art
The inflation molding technique has been widely employed in the manufacture of films made of a thermoplastic resin such as a polyolefin including a low density polyethylene and a high density polyethylene and so on, because an apparatus for conducting the inflation molding is simple and inexpensive.
FIG. 1 schematically illustrates a conventional apparatus for making a film by inflation molding. According to this apparatus, a molten thermoplastic resin is extruded through a circular die 2 of an extruder, and the extruded tubular film 4 is drawn while being cooled, while, at the same time, a gaseous medium such as air is introduced into an inner space of the tubular film so that the tubular film can be expanded to a predetermined size by the effect of the pressure of the gaseous medium. The tubular film 4 is solidified to have a final diameter and is then guided by a pair of guide members 11, constructed of flat plates arranged in a V-shape so as to guide the opposing surfaces of the tubular film to be close to each other, so that the tubular film 4 is progressively flattened. Then, the film is passed through a pair of nipping rolls to give a folded sheet, that is, a folded tubular film.
Thereafter, the folded sheet may be converted into two separate flat films with the lug of the folded sheet cut off by a slitter. The folded tubular film or the resulting flat films are then wound up around a take-up roll or reel.
With respect to the inflation molding, various attempts have been made. For example, the Japanese Patent Application Laid-open No. Hei. 9-109274 (JPA 9-109274) discloses a water quenching inflation molding in which the outer surface of the tubular film expanded to a predetermined size is rapidly cooled by water. According to this method, by adjusting the cooling speed of the tubular film, the temperature of the inner surface of the tubular film is maintained so high that the opposite portions of the inner surfaces of the tubular film are fused together as it passes through a pair of nipping rolls.
JPA 9-109274 describes that the method disclosed therein is advantageous in that the formed film can be drawn at a high speed because the blocking of the film is suppressed. In addition, the water quenching inflation molding technique generally provides a highly transparent film by a rapid cooling process.
Japanese Patent Application Laid-open No. 2001-239583 (JPA 2001-239583) discloses a method for making a highly transparent inflation film at a high speed and with good stability. According to this method, the inflation molding is so carried out as to satisfy the following conditions:a×b≧1600 anda×e≧720wherein “a” represents the thickness of the film to be molded in terms of μm, “b” represents the molding speed at which the film is molded in terms of meter per minute, and “c” represents the peak value of the cooling speed at which a molten resin is cooled in term of ° C. per second.
Japanese Patent Application Laid-open No. Hei. 10-29243 (JPA 10-29243) discloses a method for making a film by an air quenching inflation molding characterized in that an aromatic crystalline thermoplastic resin having a melt flow rate of 3 to 8 is extruded downwardly from a circular die, the temperature of which is adjusted to about equal to or lower than the melting point of such aromatic crystalline thermoplastic resin. According to JPA 10-29243, it is described that a highly transparent film can be prepared from the aromatic crystalline thermoplastic resin with low cost using an apparatus being compact in size. Also, JPA 10-29243 discloses an apparatus for making an inflation film wherein a guide member and the nipping rolls are integrated into a single unit that is movable upward and downward.
In the formation of an inflation film, it has been generally pointed out that where the raw resin is stiff, the resultant inflation film is susceptible to formation of slack and/or wrinkles, which makes the marketability of the film consequently low.